1. Field of the Invention
The invention relates to eleotrostatographic document production machines, and more particularly to the detection of the need to adjust the transfer and/or the fusing subsystems of such machines.
2. Description of the Prior Art
In electrostatographic document production machines such as printers and copiers, image contrast, and density can be adjusted by changing certain process control parameters. Such parameters most frequently include toner concentration, primary voltage V.sub.0, exposure E.sub.0, and development station electrode bias voltage V.sub.b.
U.S. Pat. No. 4,894,685, issued Jan. 16, 1990 to Shoji, discloses a method for setting color process control parameters by forming a plurality of different density test patches in a non-transfer portion of a photoconductor. Process control parameters are set in accordance with the differences between the recorded densities and aim densities. However, control of the electrostatographic process based on the toner density on the photoconductor alone does not insure optimum images if the transfer or the fusing subsystems are out of adjustment, as errors in these subsystems will not affect the density of the test patch on the photoconductor.
Some electrostatographic machines have attempted to overcome this problem by placing the density sensor downstream of the transfer or the fusing subsystems, but these machines cannot differentiate between errors caused by the transfer and/or fusing subsystems from errors in the other subsystems which would cause an error in the amount of toner applied to the test patch on the image receiver.
U.S. Pat. No. 4,277,162, which issued to Kasahara et al. on Jul. 7, 1981, teaches process control in response to the optical density of a test patch before and after transfer to a sheet-supporting member. If the density is sufficient before transfer, and insufficient after transfer, the transfer bias will be adjusted. However, Kasahara et al. do not disclose transfer and fusing of the test patches to the copy sheet before the final density reading. Thus, the process does not provide a check of the operation of the fusing subsystem. Further, transfer characteristics to a copy sheet are generally different than transfer characteristics to the sheet-supporting member. Therefore, the Kasahara et al. device may very possibly produce false readings of the transfer efficiency to a copy sheet.